The interaction of myosin with actin with the concomitant hydrolysis of ATP produces biological movement and force during muscle contraction. We propose measuring low-angle solution scattering from the enzymatically active head portion of myosin-subfragment 1 (S1). We will use cloned Dicryostelium discoideum S1's which have sited-directed mutations such that during S1's ATPase cycle it is arrested in the S1 ATP state. Such an experiment will allow us to test whether it is the binding or the hydrolysis of ATP that is responsible for energetically "cocking" S1 prior to force production.